Measuring earthquakes
Scientists use two values to describe the size of an earthquake – magnitude and intensity .
Magnitude
The magnitude of an earthquake is a measure of the total amount of energy released by the ground movement at its source. It is commonly determined by analysing the shaking recorded on several seismographs. The shaking movement, or ground acceleration, causes the trace on the seismograph to deflect up and down with time. The distance between the centre of the trace and the highest peak (or lowest trough) recorded by the seismograph is known as the maximum deflection or amplitude of the ground acceleration.
By recording the amplitude on several seismographs at different distances from the earthquake, scientists can triangulate to determine the magnitude and point of origin of the earthquake. The precise point of origin below ground where the earthquake took place is called the focus, while the location on the Earth’s surface directly above the focus is known as the epicentre.
In New Zealand, earthquake magnitude is typically measured from 0–10 on the logarithmic Moment Magnitude Scale, which is often denoted as Mw or just M. The exact relationship between magnitude and the energy released is not fully understood, but for a one-step increase in magnitude, the amount of energy released increases by roughly 32 times. For example, a magnitude 7 earthquake releases 32 times the amount of energy (and is therefore 32 times bigger) than a magnitude 6 earthquake.
The Richter scale
In 1935, American seismologist Charles Richter devised a way to measure the magnitude of an earthquake from the largest deflection recorded on a seismograph at a specific distance from a fault. The Richter scale of earthquake magnitude has values from 0–10. Richter’s method works well for small to moderate earthquakes but loses sensitivity when comparing different earthquakes with large amounts of energy released. Today, the magnitude is estimated based on the rupture energy released. This magnitude has been calibrated back to the Richter scale giving similar values to that originally obtained by Richter for magnitudes less than 6 and greater values for larger magnitude events.
Intensity
The intensity of an earthquake is a way to describe the degree of ground movement at a specific site during a specific event. Two forms of intensity are used – objective and subjective intensity.
Objective intensity
Objective intensity (sometimes called instrumental intensity) is a quantitative measure of how strongly the earthquake’s energy manifests on the ground. It is based on seismic data collected by instruments within the earthquake zone.
Objective intensity is commonly measured using peak ground acceleration (PGA) or peak ground velocity (PGV), although other measures, such as duration, are also used.
Peak ground acceleration, which is a measure of earthquake shaking on the ground, can be expressed in vertical (PVA) and horizontal (PHA) components. It is widely used by engineers to express the intensity of ground shaking a building must be designed to withstand without collapse.
Subjective intensity
Subjective intensity (sometimes called felt or observed intensity) is a qualitative measure of how strongly the earthquake’s energy manifests on the ground. It is based on the earthquake’s observable effects on people, buildings and other objects at different distances from the epicentre. Effects usually become less pronounced the further the observer is located from the earthquake, although local geological and topographical effects can cause localised magnification. Therefore, the measure of felt intensity can vary widely over the area affected, with high intensities near the epicentre and lower values further away.
In this country, reports of earthquake intensity are measured on a scale from 0–12 on the New Zealand Modified Mercalli Intensity scale, abbreviated as MMI or MM.
Geonet, New Zealand’s national geophysical monitoring system, provides the following description of the MM scale and the typical effects that would be observed at each level of intensity .
|
Level |
Description |
|
MM1 |
People: Not felt except by a very few people under exceptionally favourable circumstances. |
|
MM2 |
People: Felt by persons at rest, on upper floors or favourably placed. |
|
MM3 |
People: Felt indoors; hanging objects may swing, vibration similar to passing of light trucks, duration may be estimated, may not be recognised as an earthquake. |
|
MM4 |
People: Generally noticed indoors but not outside. Light sleepers may be awakened. Vibration may be likened to the passing of heavy traffic, or to the jolt of a heavy object falling or striking the building. Fittings: Doors and windows rattle. Glassware and crockery rattle. Liquids in open vessels may be slightly disturbed. Standing motorcars may rock. Structures: Walls and frames of buildings, and partitions and suspended ceilings in commercial buildings, may be heard to creak. |
|
MM5 |
People: Generally felt outside, and by almost everyone indoors. Most sleepers awakened. A few people alarmed. Fittings: Small unstable objects are displaced or upset. Some glassware and crockery may be broken. Hanging pictures knock against the wall. Open doors may swing. Cupboard doors secured by magnetic catches may open. Pendulum clocks stop, start, or change rate. Structures: Some windows Type I cracked. A few earthenware toilet fixtures cracked. |
|
MM6 |
People: Felt by all. People and animals alarmed. Many run outside. Difficulty experienced in walking steadily. Fittings: Objects fall from shelves. Pictures fall from walls. Some furniture moved on smooth floors, some unsecured free-standing fireplaces moved. Glassware and crockery broken. Very unstable furniture overturned. Small church and school bells ring. Appliances move on bench or table tops. Filing cabinets or “easy glide” drawers may open (or shut). Structures: Slight damage to Buildings Type I. Some stucco or cement plaster falls. Windows Type I broken. Damage to a few weak domestic chimneys, some may fall. Environment: Trees and bushes shake, or are heard to rustle. Loose material may be dislodged from sloping ground, e.g. existing slides, talus slopes, shingle slides. |
|
MM7 |
People: General alarm. Difficulty experienced in standing. Noticed by motorcar drivers who may stop. Fittings: Large bells ring. Furniture moves on smooth floors, may move on carpeted floors. Substantial damage to fragile contents of buildings. Structures: Unreinforced stone and brick walls cracked. Building Type I cracked with some minor masonry falls. A few instances of damage to Building Type II. Unbraced parapets, unbraced brick gables, and architectural ornaments fall. Roofing tiles, especially ridge tiles may be dislodged. Many unreinforced domestic chimneys damaged, often falling from roof-line. Water tanks Type I burst. A few instances of damage to brick veneers and plaster or cement-based linings. Unrestrained water cylinders (water tanks Building Type II) may move and leak. Some windows Type II cracked. Suspended ceilings damaged. Environment: Water made turbid by stirred up mud. Small slides such as falls of sand and gravel banks, and small rock falls from steep slopes and cuttings. Instances of settlement of unconsolidated or wet, or weak soils. Some fine cracks appear in sloping ground. A few instances of liquefaction (i.e. small water and sand ejections). |
|
MM8 |
People: Alarm may approach panic. Steering of motorcars greatly affected. Structures: Buildings Type I heavily damaged, some collapse. Building Type II damaged, some with partial collapse. Building Type III damaged in some cases. A few instances of damage to Structures Type IV. Monuments and pre-1976 elevated tanks and factory stacks twisted or brought down. Some pre-1965 infill masonry panels damaged. A few post-1980 brick veneers damaged. Decayed timber piles of houses damaged. Houses not secured to foundations may move. Most unreinforced domestic chimneys damaged, some below roof-line, many brought down. Environment: Cracks appear on steep slopes and in wet ground. Small to moderate slides in roadside cuttings and unsupported excavations. Small water and sand ejections and localised lateral spreading adjacent to streams, canals, lakes, etc. |
|
MM9 |
Structures: Many Buildings Type I destroyed. Buildings Type II heavily damaged, some collapse. Buildings Type III damaged, some with partial collapse. Structures Type IV damaged in some cases, some with flexible frames seriously damaged. Damage or permanent distortion to some Structures Type V. Houses not secured to foundations shifted off. Brick veneers fall and expose frames. Environment: Cracking of ground conspicuous. Landsliding general on steep slopes. Liquefaction effects intensified and more widespread, with large lateral spreading and flow sliding adjacent to streams, canals, lakes, etc. |
|
MM10 |
Structures: Most Buildings Type I destroyed. Many Buildings Type II destroyed. Buildings Type III heavily damaged, some collapse. Structures Type IV damaged, some with partial collapse. Structures Type V moderately damaged, but few partial collapses. A few instances of damage to Structures Type VI. Some well-built timber buildings moderately damaged (excluding damage from falling chimneys). Environment: Land slides very widespread in susceptible terrain, with very large rock masses displaced on steep slopes. Landslide dams may be formed. Liquefaction effects widespread and severe. |
|
MM11 |
Structures: Most Buildings Type II destroyed. Many Buildings Type III destroyed. Structures Type IV heavily damaged, some collapse. Structures Type V damaged, some with partial collapse. Structures Type VI suffer minor damage, a few moderately damaged. |
|
MM12 |
Structures: Most Building Type III destroyed. Structures Type IV heavily damaged, some collapse. Structures Type V damaged, some with partial collapse. Structures Type VI suffer minor damage, a few moderately damaged. |
The MM scale uses the following definitions.
- Buildings Type I: Buildings with low standard of workmanship, poor mortar or constructed of weak materials like mud brick or rammed earth. Soft storey structures (e.g. shops) made of masonry, weak reinforced concrete or composite materials (e.g. some walls timber, some brick) not well tied together. Masonry buildings otherwise conforming to buildings Types I to III but also having heavy unreinforced masonry towers. (Buildings constructed entirely of timber must be of extremely low quality to be Type I.)
- Buildings Type II: Buildings of ordinary workmanship with mortar of average quality. No extreme weakness, such as inadequate bonding of the corners, but neither designed nor reinforced to resist lateral forces. Such buildings not having heavy unreinforced masonry towers.
- Buildings Type III: Reinforced masonry or concrete buildings of good workmanship and with sound mortar but not formally designed to resist earthquake forces.
- Structures Type IV: Buildings and bridges designed and built to resist earthquakes to normal use standards, i.e. no special collapse or damage-limiting measures taken (mid-1930s to c. 1970 for concrete and to c. 1980 for other materials).
- Structures Type V: Buildings and bridges designed and built to normal use standards, i.e. no special damage-limiting measures taken, other than code requirements, dating from since c. 1970 for concrete and c. 1980 for other materials.
- Structures Type VI: Structures dating from c. 1980 with well defined foundation behaviour, which have been specially designed for minimal damage, e.g. seismically isolated emergency facilities, some structures with dangerous or high-value contents or new-generation low-damage structures.